This type of photosensitive sensor is being integrated more and more frequently in motor vehicles. The images obtained can be processed in order to display images on screens, for example at the dashboard or projected on the windscreen, in particular to alert the driver in the case of danger or simply to improve his visibility. They can also participate in the automatic triggering of a functionality of the vehicle (automatic triggering of braking, visual or audible warning, control of certain functions of the headlight, etc). These sensors can be used by day. They can also be used at night, and then their capacities for detecting radiation in the infrared range are rather exploited. In nocturnal use, these sensors thus participate for example in the “night vision” functionality, where the images, once processed, are projected onto a display screen generally in the form of black and white images.
However, it can scarcely be envisaged using as it stands the same sensor installed on a vehicle in daytime use and in nocturnal use, even if in principle it may be capable of detecting in both the visible and infrared ranges. This is because, if a sensor sensitive to infrared is used so that it can “see” at night, by day the reflection of the sun on the road in the areas close to the vehicle will easily cause saturation of it in the infrared: this saturation tends to make it inappropriate for daytime use, with a loss of quality of the images contained. (In the present text the word image will be used in the broad sense, namely any signal or set of signals received or restored by the photosensitive sensor.)
In order to have images of the required quality with these types of sensor both by day and by night and whatever the sunshine, there have up to the present time been two solutions.
The first solution consists of using two sensors, a first sensor dedicated to daytime use and equipped with a filter “cutting” the infrared in order to avoid any saturation in the case of sunshine and a second sensor dedicated to nocturnal use. The drawback of this first solution is that two sensors are required, which increases the cost, complexity and size of the system using the images collected by the sensors.
The second solution consists of using a single sensor, and equipping it with a removable filter able to filter the infrared: in daytime use with strong sunshine, the filter is active and prevents the saturation of the sensor with infrared. On the other hand, the filter is removed in nocturnal use and in daytime use with weak sunshine. This solution is also not devoid of drawbacks: providing a movable part in a sensor is complex and presents a risk of malfunctioning. In addition, it is necessary to provide means for detecting the day/night/degree of sunshine environmental conditions, means that are complex, expensive and liable to failure.
The aim of the invention is therefore to remedy the drawbacks of the existing solutions. More precisely, the aim of the invention is to develop novel photosensitive sensors able to provide a satisfactory image quality both by day and by night, and which are in particular simpler and/or more reliable.